docstr: Tests E[F(t)] == F_0.
docstr_tok: ["Tests","E","[","F","(","t",")","]","==","F_0","."]
code: def test_drift(self, beta, volvol, rho, time_step):
      """Tests E[F(t)] == F_0."""
      dtype = tf.float64
      times = [0.1, 1.0]
      num_samples = 10
      initial_forward, initial_volatility = 1.0, 0.1
      test_seed = [123, 124]
  
      process = SabrModel(
          beta=beta,
          volvol=volvol,
          rho=rho,
          dtype=dtype,
          enable_unbiased_sampling=True,
      )
      paths = process.sample_paths(
          initial_forward=initial_forward,
          initial_volatility=initial_volatility,
          times=times,
          time_step=time_step,
          num_samples=num_samples,
          seed=test_seed,
          random_type=tff.math.random.RandomType.STATELESS_ANTITHETIC,
      )
  
      paths = self.evaluate(paths)
      for i in range(len(times)):
          mean = np.mean(paths[:, i, 0])
          self.assertAllClose(mean, initial_forward, rtol=0.1, atol=0.1)
  
file_path: tf-quant-finance/tf_quant_finance/models/sabr/sabr_model_test.py
imports: import math
  from absl.testing import parameterized
  import numpy
  import tensorflow.compat.v2
  import tf_quant_finance
  from tensorflow.python.framework import test_util
  from tf_quant_finance.models import euler_sampling
label_desc: Use image node_id 2 for calling the SabrModelTest obj's underlying member method code with example usage: obj.test_drift(beta, volvol, rho, time_step) without return types